She worked as a researcher at SNP headquarters from 1977 to 1979 and was a member of the left-wing79 Group inside the SNP during the early 1980s, but avoided expulsion as she was not a member of its steering committee (future SNP leader Alex Salmond by contrast who served on the 79 Group committee was expelled, whilst Margo MacDonald resigned from the party in protest before she could be expelled).

Cunningham first came to prominence in 1995 when she won the bitter Perth and Kinrossby-election to replace the Conservative MP, Sir Nicholas Fairbairn, who had died. She had initially been left off the SNP's candidate shortlist over her brief relationship in the 1970s with Donald Bain, the former husband of SNP stalwart Margaret Ewing, on the grounds that the issue could prove an embarrassment to the party. However, she was put back in contention following an intervention by the then party leader Alex Salmond, and after Ewing made clear she had no objection to Cunningham's candidature.

In 1999 she became the MSP for Perth and continues to represent the area in the Scottish Parliament to this day. In 2000 she was elected the SNP Senior Vice-Convener (depute leader). Also in that year she helped establish the Scottish Left Review publication. She stood down as an MP in 2001, to concentrate on the Scottish Parliament.

John Swinney announced his resignation as leader of the SNP on 22 June 2004, and on the same day Roseanna Cunningham announced that she would be a candidate in the subsequent election for the party leadership. In the early stages of the campaign she appeared to be the clear front-runner, but the entirely unexpected decision of the popular former leader Alex Salmond to enter the race just before nominations closed changed everything, and Cunningham ultimately finished a distant second.

In December 2006 she led an unsuccessful attempt to prevent same-sex couples gaining the right to adopt children,[1] despite having previously been named ScotsGay Parliamentarian of the Year in 1998.[2] When legislation to introduce same-sex marriage in Scotland was passed by the Scottish Parliament in February 2014, she had voted against the bill.[3]

During the Scottish independence referendum, 2014 Cunningham posted a photograph on Twitter of a letter written by Lieutenant General Sir Norman Arthur, which asked for donations to Better Together but also contained his home and email addresses and phone number. Following this, Arthur received several abusive emails from cybernats, which he complained to the Scottish Government about.[5]